I used to hunt in the rain and in swamps a lot, and used Gametracker and Saunder's String Trackers on 24 recovered deer. The string trackers never saved me a deer that I would not have otherwise found - but they sure saved me a lot of tracking time. But I also had a few deer 'jump' the string, and I felt the extra noise from the string trackers cost me. I would not recommend them.
I now have a blood trailing dog - she's more fun than a string tracker, lol.
-fsh